1. Field of the Invention
The present invention relates to an image recording apparatus having an image recording means for recording a two-dimensional image on a recording sheet such as a photosensitive medium held on the outer or inner circumferential surface of a drum, an a method of generating a pixel clock, which is preferably applicable to such an image recording apparatus.
2. Description of the Related Art
There have heretofore been known external surface scanning light beam image recording apparatus for recording a two-dimensional image on the entire surface of a recording medium on the outer circumferential surface of a cylindrical drum by rotating the drum, scanning the recording medium with an intensity-modulated light beam emitted from an optical system in a main scanning direction, and moving the optical system in an axial direction of the drum thereby to scan the recording medium in an auxiliary scanning direction transverse to the main scanning direction. See, for example, Japanese laid-open patent publications Nos. 5-207250, 9-149211, and 10-16290, for details.
The drum, which has a diameter of 300 mm and a length of 1 m and is made of aluminum or the like, of those disclosed external surface scanning light beam image recording apparatus actually suffers various dimensional errors. For example, the drum has various diameter and outer circumferential surface dimension variations, which fall within a machining tolerance range, caused in the manufacturing process, and also have eccentricity errors introduced when the drums are assembled. Consequently, even when the drum is rotated at a constant speed, the circumferential speed of the outer circumferential surface of the drum is not constant. With the irregular circumferential speed, when an image is plotted on the photosensitive medium by the light beam that is intensity-modulated, e.g., selectively turned on and off, with pixel clock pulses at constant intervals, the recorded image tends to be unduly expanded or contracted in local regions.
One solution proposed in the known apparatus has been to measure a distortion of an image which has been plotted with pixel clock pulses and correcting the spaced intervals of the pixel clock pulses when the image is actually recorded for thereby minimizing expansions and contractions of the image. According to the system disclosed in Japanese laid-open patent publication No. 5-207250, the frequency-diving ratio of a PLL circuit which generates pixel clock pulses is varied to correct the spaced intervals of the pixel clock pulses. However, the disclosed solution is disadvantageous in that the image tends to be distorted due to a pull-in time of the PLL circuit at the time the frequency-diving ratio thereof is varied.
The technique revealed in Japanese laid-open patent publication No. 9-149211 corrects the spaced intervals of pixel clock pulses by changing an input voltage applied to a voltage-controlled oscillator. The revealed technique is also problematic in that the image is liable to suffer a new distortion owing to the temperature characteristics of the voltage-controlled oscillator.
It has been proposed to use a programmable delay line or a plurality of delay lines to correct clock pulse positions for solving the problem disclosed in Japanese patent laid-open publication No. 5-207250 or Japanese laid-open patent publication No. 9-149211. However, a correcting circuit made of inexpensive delay line or lines fails to achieve a required level of accuracy and resolution.
To eliminate the above difficulties, the system disclosed in Japanese laid-open patent publication No. 10-16290 employs, as shown in FIGS. 13 and 14 of the accompanying drawings, a rotary encoder 1 mounted on the shaft of a motor for rotating the drum to generate a fundamental clock whose frequency is multiplied to produce an original clock by a PLL circuit 2. The pulses of the original clock are digitally counted by a counter 3. The counter 3 comprises a preset down counter and functions as a frequency divider, and is also referred to as a frequency divider. Based on the count from the counter 3, a CPU 4 reads correcting data from a correcting data memory 5. Based on the read correcting data, a control circuit 6 selects a frequency-dividing ratio of the counter or frequency divider 3 to divide the frequency of the original clock from the PLL circuit 2 by 7, 8, or 9.
The disclosed system can achieve a required level of accuracy and resolution because the clock pulse positions are corrected digitally by the counter 3 and a clock adjusting means 7 which is made up of the CPU 4, the corrective data memory 5, and the control circuit 6.
The correcting data are produced as follows: The circumferential surface of the drum that corresponds to a full image surface is developed into a flat rectangular surface, which is divided along main and auxiliary scanning directions into a mesh pattern of small rectangular cells or grip points, and correcting data for the respective rectangular cells or grid points are stored as original correcting data in the correcting data memory 5. The CPU 4 calculates, from coordinates to be recorded next that are obtained by counting pixel clock pulses and the stored original correcting data, correcting data for the coordinate position to be recorded next, and determines a recording time based on the calculated correcting data.
However, the above technique is disadvantageous in that when an exposure recording condition such as a dot per inch (DPI) with respect to the photosensitive medium is changed, it is necessary to calculate and regenerate original correcting data for respective grid points of the full image surface, and hence the productivity is greatly reduced.
The foregoing drawback may be eliminated by generating original correcting data for respective grid points of the full image surface with respect to each exposure recording condition and storing the generated original correcting data in the correcting data memory. This approach is highly costly because a large-storage-capacity semiconductor memory or a hard disk is needed as the correcting data memory for storing such original correcting data.
If clock pulse positions are to be corrected in view of the expansion or contraction of the drum due to environmental temperature changes, then it is necessary to store original correcting data for each temperature, resulting in a possible further increase in the cost. The system shown in FIGS. 13 and 14 is also problematic in that it requires a complex control process for the control circuit 6 to set frequency-dividing ratios in the counter or frequency divider 3 for small variations of clock pulse positions to be corrected, the CPU 4 requires a considerable power to generate a correcting table for setting frequency-dividing ratios, and the correcting data memory 5 needs a large storage capacity for storing the calculated data.
In addition, the original clock outputted from the PLL circuit 2, whose frequency is 8 times the frequency of the pixel clock, is usually frequency-divided by 8 and partly frequency-divided by 7 or 9 by the counter or frequency divider 3, for the correction of pixel clock positions. Therefore, pixel clock positions are corrected in fixed positions along the main scanning direction at all times, so that an image produced on the photosensitive medium tends to suffer a quality degradation such as a striped irregularity or a moiré pattern.